SYDNEY (AFP) ― The swathe of Indian Ocean where acoustic transmissions were detected was Thursday ruled out as the crash zone of missing Malaysia Airlines Flight MH370 after a lengthy underwater search.

The Australian-led Joint Agency Coordination Center said that the search in the area using a mini sub where the ping-emitting beacons were believed detected in early April was now complete.

“The Joint Agency Coordination Center can advise that no signs of aircraft debris have been found by the autonomous underwater vehicle since it joined the search effort,” JACC said. It added that the Australian Transport Safety Bureau had advised that “the area can now be discounted as the final resting place of MH370.”

Australian ship Ocean Shield, which is carrying the U.S. Bluefin-21 submersible, has now left the area after the sub scoured 850 square kilometers of ocean for the jet that vanished on March 8 carrying 239 people.

The end of the underwater mission came as the U.S. Navy’s deputy director of ocean engineering Michael Dean told CNN that the pings at the heart of the search were no longer believed to have come from the plane’s black box.

A U.S. Navy spokesman later said his comments were “speculative and premature.” JACC did not comment.

JACC said the operation would now move to the next phase involving sophisticated equipment to scan the unmapped ocean bed with all existing information and analysis reviewed to define a search zone of up to 60,000 square kilometers.

A Chinese survey ship, Zhu Kezhen, is currently mapping areas of the sea floor in preparation for the commercially contracted deep ocean search which is expected to begin in August and take up to 12 months, JACC said.